The disclosure relates generally to investment casting, and more particularly, to a mold system for forming a casting article for investment casting in which the mechanical integrity of a core (e.g., a ceramic core) can be tested by viscosity manipulation.
Investment casting is used to manufacture a large variety of industrial parts such as turbomachine blades. Investment casting uses a casting article having a sacrificial material pattern to form a ceramic mold for the investment casting. Certain types of casting articles may include a core or insert within the sacrificial material pattern. The core defines an interior structure of the component and becomes a part of the ceramic mold used during the investment casting. The core can include a large variety of intricate features that define an interior structure of the component. Cores can be additively manufactured to allow for rapid prototyping and manufacturing of the cores. The casting article is made by molding a sacrificial material fluid, such as hot wax or a polymer, about the core in a mold that defines the shape of the component surrounding the core. The hardened sacrificial material formed about the core defines the shape of the component for the investment casting.
Each casting article, either individually or in a collection thereof, can be dipped in a slurry and coated with a ceramic to form a ceramic mold for the investment casting. Once the sacrificial material is removed from the ceramic mold, the mold can be used to investment cast the component using a molten metal, e.g., after pre-heating the ceramic mold. Once the molten metal has hardened, the ceramic mold can be removed, and the core can be removed using a leachant. The component can then be finished in a conventional fashion, e.g., heat treating and conventional finishing.
Investment casting is a time consuming and expensive process, especially where the component must be manufactured to precise dimensions. In particular, where precise dimensions are required, formation of the casting article must be very precise. Each mold used to form the casting article can be very costly, and can take an extensive amount of time to manufacture. Consequently, any changes in the core or the component can be very expensive and very time consuming to address. Other challenges that can be costly and time consuming to address are unforeseen weaknesses in the core that cause it to crack or break either during formation of the casting article (e.g., during casting of the sacrificial material about the core), or during the actual investment casting. For example, high pressure sacrificial fluid injected into a mold about the core during casting article formation can crack or break the core, or molten metal injected during the investment casting can crack or break the core. In the former case, the core must be adjusted, and in the latter case, the core and/or the casting article mold may need adjusting. In any event, the changes are costly and time consuming. Currently, there is no mechanism to proactively address the core cracking/breaking challenges.
One approach to reduce time and costs employs additive manufacture of the cores and molds for making the casting article. In particular, additive manufacture allows for more rapid turnaround for design changes in cores and/or the component leading up to the component manufacturing steps. Additive manufacturing (AM) includes a wide variety of processes of producing an object through the successive layering of material rather than the removal of material. Additive manufacturing can create complex geometries without the use of any sort of tools, molds or fixtures, and with little or no waste material. Instead of machining objects from solid billets of material, much of which is cut away and discarded, the only material used in additive manufacturing is what is required to shape the object. Current categories of additive manufacturing may include: binder jetting, material extrusion, powder bed infusion, directed energy deposition, sheet lamination and vat photopolymerization.
Additive manufacturing techniques typically include taking a three-dimensional (3D) computer aided design (CAD) file of the object (e.g., core and/or casting article mold) to be formed, electronically slicing the object into layers (e.g., 18-102 micrometers thick) to create a file with a two-dimensional image of each layer (including vectors, images or coordinates) that can be used to manufacture the object. The 3D CAD file can be created in any known fashion, e.g., computer aided design (CAD) system, a 3D scanner, or digital photography and photogrammetry software. The 3D CAD file may undergo any necessary repair to address errors (e.g., holes, etc.) therein, and may have any CAD format such as a Standard Tessellation Language (STL) file. The 3D CAD file may then be processed by a preparation software system (sometimes referred to as a “slicer”) that interprets the 3D CAD file and electronically slices it such that the object can be built by different types of additive manufacturing systems. The object code file can be any format capable of being used by the desired AM system. For example, the object code file may be an STL file or an additive manufacturing file (AMF), the latter of which is an international standard that is an extensible markup-language (XML) based format designed to allow any CAD software to describe the shape and composition of any three-dimensional object to be fabricated on any AM printer. Depending on the type of additive manufacturing used, material layers are selectively dispensed, sintered, formed, deposited, etc., to create the object per the object code file.
One form of powder bed infusion (referred to herein as metal powder additive manufacturing) may include direct metal laser melting (DMLM) (also referred to as selective laser melting (SLM)). This process is advantageous for forming metal molds for forming casting articles. In metal powder additive manufacturing, metal powder layers are sequentially melted together to form the object. More specifically, fine metal powder layers are sequentially melted after being uniformly distributed using an applicator on a metal powder bed. Each applicator includes an applicator element in the form of a lip, brush, blade or roller made of metal, plastic, ceramic, carbon fibers or rubber that spreads the metal powder evenly over the build platform. The metal powder bed can be moved in a vertical axis. The process takes place in a processing chamber having a precisely controlled atmosphere. Once each layer is created, each two dimensional slice of the object geometry can be fused by selectively melting the metal powder. The melting may be performed by a high powered irradiation beam, such as a 100 Watt ytterbium laser, to fully weld (melt) the metal powder to form a solid metal. The irradiation beam moves or is deflected in the X-Y direction, and has an intensity sufficient to fully weld (melt) the metal powder to form a solid metal. The metal powder bed may be lowered for each subsequent two dimensional layer, and the process repeats until the object is completely formed. In order to create certain larger objects faster, some metal additive manufacturing systems employ a pair of high powered lasers that work together to form an object, like a mold. Other additive manufacturing processes, such as 3D printing, may form layers by dispensing material in layers.
Although additive manufacturing of cores and/or molds for casting article formation has reduced time and cost for adjusting cores and/or molds, challenges remain. Most notably, current mold systems and practices for forming a casting article form one mold regardless of variations in cores. When variations in cores are subtle or when the core has fine or intricate features, it can result in cracked or broken cores and/or imprecise casting articles. Where variations in cores are more profound, e.g., where they share a common structure but also have other structure that varies widely to build different components, each variation of core must have its own mold. Current mold systems used for forming the casting articles are also not sufficiently thermally adjustable to accommodate sacrificial material fluid flow across different cores.
Another challenge with current investment casting is ensuring cores within a casting article can withstand the actual investment casting, i.e., the casting of a molten metal about the core. The current practice includes a trial and error approach in which a casting article is used to perform an investment casting to determine its efficacy. During investment casting, the core may, for example, break, crack or prevent adequate molten metal flow to form the component. In the absence of any mechanism to predict core efficacy, when a problem is identified during investment casting, changes to the core, the metal casting article mold, and/or the casting article formation process must be made, all of which are time consuming and expensive.